Telescope House September Sky Guide Part 2

Saturn

At just past opposition (reached on August 27th), Saturn is ideally, placed for observation throughout the night. Currently found in the constellation of Aquarius, Saturn presents a visual magnitude of +0.4 and an apparent size of 19 arc seconds across at the month’s beginning. Saturn rises at just before 8 pm on the 1st, transits a little before 1 am the following morning and sets at around 6 am.

The early morning of the 1st also sees an interesting Saturnian transit event, when the moon Tethys moves in front of Saturn’s disk. Transits of Saturn’s moons are a much more challenging event to observe than those of Jupiter’s Galilean satellites. You will need a telescope of probably around 10 inches in aperture, as a minimum - and it is much more likely that you will see the shadow of the moon’s transit, rather than the moon itself, against Saturn’s atmosphere. Conditions will have to be very clement and the atmosphere extremely steady to make observations of this sort. However, the morning of the first see Saturn at a reasonable altitude above the horizon and while it would require an alignment of favourable circumstances and pretty healthy magnification, for those with the prerequisite equipment, observing skill, and patience, is not impossible.

By the time we get to mid September, Saturn will have faded very fractionally to a visual magnitude +0.5. The planet will rise at a little before 7 pm and transit at just before midnight. It will set around 5 am the following morning.

At the end of the month, Saturn will faded a little further 2+0.6 magnitude and now displays and 18.6 ox second diameter disc. The planet rises a little before 6 pm, transiting at a little before 11 in the evening and sitting at just before 4 am the following morning. Post-opposition is always the most favourable time for observation of an outer planet in the evening and we are heading towards this window of observational opportunity as far as Saturn is concerned. Anyone with a telescope is thoroughly encouraged to make the most of this opportunity. Like Jupiter, Saturn is now rising from the doldrums of the southern part of the ecliptic, which it has occupied for the best part of a decade now. While it is nowhere near as high in the sky as its neighbour, Jupiter, the trend for northern hemisphere observation is definitely upwards as far as Saturn is concerned. Standing just under 27° high at transit point on the 30th, Saturn presents a very reasonable target for northern hemisphere observers. Time spent at the eyepiece observing Saturn is rarely wasted - so make the most of it.

Uranus and Neptune

Of the two out gas giants, the focus is predominantly on Neptune this month, as it comes to opposition on 19th September. In truth, oppositions of both outer worlds, never result in a massive boost in brightness, as they are both so far away from us here on Earth, even at the best of times. The 19th will find Neptune a resident in Pisces at a visual magnitude of +7.8 and displaying a disc of 2.4 seconds diameter.

Neptune was first identified through mathematical prediction. While Galileo observed Neptune near Jupiter in 1612 and 1613, he mistook it for a star. After Sir William Herschel's discovery of Uranus in 1781, anomalies in the observation of the planet’s orbit hinted at another distant planet, further out in the icy reaches of the solar system. English astronomer John Couch Adams and French mathematician Urbain Le Verrier separately calculated its potential location. Their predictions, initially overlooked, were later recognised for their accuracy. However, due to outdated star maps and communication issues, Neptune was observed, but not identified by the British. Meanwhile, Le Verrier shared his predictions with the Berlin Observatory. Resident Astronomers Johann Galle and Henrich d’Arrest found Neptune within an hour of searching on 24th September 1846, very close (within a degree) of Le Verrier’s predicted location.

Neptune’s prominent blue colouration is often remarked upon. When you find it in a telescope, or larger binoculars, it is faint, but pretty unmistakable, often being described as looking akin to a planetary nebula itself. Around opposition gives us the best opportunity to observe the solar system’s (current) last “true” planet.

Further east in the ecliptic, lies Neptune’s neighbour, Uranus. A resident of Aries currently, the prominent planet, Jupiter situated around seven and three-quarter degrees to the west of Uranus acts as a handy signpost for its location. At +5.7 magnitude, Uranus is technically a naked eye object, but is much easier to find in binoculars. Its greeny-grey disc, again, looks quite similar to a brighter planetary nebula and at 3.7 arc seconds diameter it is similar in size to many. Given a significantly large telescope (usually 12 inches or above in aperture), it is possible to visually observe some striation on Uranus’ disk, during times of good seeing. Experienced observers have managed to glimpse some subtle albedo features with smaller telescopes, but have doubtlessly been extremely patient and methodical in their observation techniques. As Uranus is now the most northerly of all the planets in the sky, northern hemisphere observers are encouraged to make the most of its position. As Uranus transits during September, it will stand around 57 1/2° above the horizon (as seen from 51° north).

Comets

A newly-discovered comet, C/2023 P1 (Nishimura) has been causing some speculation and it looks like it may be observable during September. Very early in the month, the comet may be observable in the early morning sky, predawn. However, it will loop north of the Sun around mid-September and will likely be unobservable during this period - unless something remarkable happens. Once the comet reemerges from the Sun’s glare in the latter half of the month, it will be an evening target.  Current peak magnitude predictions put it as high as second magnitude, though this is likely to occur when the comet is very close to the Sun, making it very difficult to observe. Still, this is an interesting object and we encourage those up early enough in the early part of the month to see if they can find it using larger binoculars. The comet may be easier to observe post-perihelion in the latter part of the month, but it remains to be seen exactly how this comet will manifest itself.

The arrival of Pegasus and Aquarius in the evening skies are a sure sign of the approach of Autumn.  These two fairly large constellations share a border and are home to some easy - and not-so-easy - deep sky targets.   

Though lacking in major nebulae, Pegasus is a haven for galaxies - maybe not quite to the extent of the Virgo and Leo regions - but has many extra-galactic targets worth attention.

The most famous feature of Pegasus is readily observable without a telescope - this is, of course, the famous Square of Pegasus. Consisting of the stars Alpheratz (Arabic for “the navel”), Scheat (”the leg”), Algenib (”the flank”), Markab (”the saddle”), the Square of Pegasus dominates this area of sky and can be used as a useful “jumping off” guide for starhopping. However, the Square of Pegasus is not solely “of Pegasus”, as Alpheratz is actually now officially a part of neighbouring Andromeda. This is a similar situation to Elnath (Beta Tauri) which is officially now part of Taurus, but has been shared as Gamma Aurigae with neighbouring Auriga. These constellations are rare as they are still shown on modern star charts as connected via their “shared” star.

A third of the way along the line between the lower stars of the Square, Markab and Algenib, lies an object not visible to the naked eye at all. This is the notable (if unspectacular) Pegasus Dwarf Galaxy, This is an associated galaxy with the nearby M31, the Andromeda Spiral and as such a neighbour of our own Milky Way. It’s a rather faint object at +13.2 mag and spread out over a reasonable area of sky, so is only really detectable in long duration photos. Dwarf galaxies are often (though not always) older, more primitive than galaxies such as our own. However, whilst they are not brilliant in the conventional visual sense, dwarf galaxies such as the Pegasus Dwarf are havens for Dark Matter. The Pegasus Dwarf lies 3 million light years away from the Milky Way and is tidally interactive with M31.

Much more easily-observed and better-known is an object on the other side of Pegasus: the great globular cluster, M15. Found 4 degrees north-east of the star Enif (Arabic for “nose”), or Epsilon Pegasi, M15 is a glorious object in any telescope or binoculars and at +6.2 mag can be seen as a naked eye object from a reasonable site. This globular was discovered by Maraldi in September 1746 and catalogued 18 years later by Messier in 1764. Located about 33600 light years away, M15 contains around 100,000 stars. As a well-known object, it has been studied exhaustively and found to contain the first extra-galactic planetary nebula discovered: Pease 1, first identified in 1928. In addition to Pease 1, M15 has a pair of co-orbiting neutron stars, 8 pulsars and two strong X-ray sources. It has been postulated that one of these sources is in fact a Black Hole, to which has been attributed M15’s relatively recent core collapse. Globular clusters are both beautiful and intriguing objects and M15 is almost certain to contain more as-yet-undiscovered features.

Back inside the Square of Pegasus lies the lovely NGC7814 - the “Little Sombrero” (so called because it resembles the Sombrero Galaxy, M104, in Virgo). NGC7814 is a Spiral, presented edge-on to our line of sight. This reveals a dark dust lane bisecting a bright core. At +10.6 mag this galaxy isn’t overly bright, but due to its compact nature, is still well-seen in small telescopes. NGC7814 is easily found due to its proximity to Algenib. 

Another galaxy near to a member of The Square is NGC7479, which lies just under 3 degrees south of Markab. This is one of the most photogenic Barred Spirals in the sky, lying almost face on to us. It was discovered in 1784 by William Herschel and is just slightly fainter than 7814 at +10.9 mag. NGC7479 is a very active galaxy - a so-called Seifert Type, in which enormous amounts of star formation are taking place. The serpentine structure of NGC7479 is beautifully depicted in long-duration photos - it almost seems to be slithering like a Sidewinder through space! 

Further north are a fascinating collection of galaxies: the NGC7331 group and Stephan’s Quintet. These two groups of galaxies are separated by just half a degree of sky and can be found north of Matar (Eta Pegasi). Of the two groups, the NGC7331 group are the more conspicuous and their principle member was discovered first - by William Herschel - in 1784. This principle galaxy, NGC7331, was thought to be a very similar size, mass and taxonomy to our own Milky Way: a tightly-barred spiral. However, most up-to-date surveys of the Milky Way suggest that it may only have two massive spiral arms, whereas NGC7331 has more (NGC6744 in Pavo is now seen to be the nearest Milky Way analogue). Behind NGC7331 lie NGCs 7340, 7336, 7335, 7327 and 7338 - some of which can be seen with averted vision in reasonable-size telescopes. NGC7331 at +9.5 mag is by far and away the most prominent of the group and can be seen in smaller scopes. The whole group is a great target for astrophotography as regular contributor Mark Blundell’s picture below clearly shows.

The second of these two galaxy groups is the famous Stephan’s Quintet. Discovered in 1877 at Marseilles Observatory by Eduoard Stephan, the Quintet consists of NGCs 7317, 7318, 7318A, 7318B, 7319 and 7320 (this is technically a Sextet as 7318A and B are separate galaxial cores). Stephan’s Quintet occupies a tiny area of 3.5‘ x 3.5’ of sky and is an area of both enormous destruction, as the component galaxies literally rip each other apart and massive areas of creation where the resulting gas-rich loops of material released by these dynamics leads to starbirth.

Of the components of the Quintet, NGC7320 appears to be an unrelated foreground object - much closer to us at 39 million light years distance as opposed to the 210-350 million light years of the other members.

Moving south into the Zodiacal constellation of Aquarius, the Water Carrier, we are presented with a large, but quite a barren area of sky. Although Aquarius is rather muted in terms of brighter stars, it is a haven for deep sky objects. The most northerly of these is the very fine globular cluster M2. At +6.46 mag, it is amongst the brighter of these interesting objects, lying 37,500 light years away from us and about 175 light years in diameter. From Earth, it appears 2.1 arc minutes in diameter, M2 is about the same relative size and brightness of the neighbouring M15 and the second of Hercules’ well-known globulars, M92. Discovered by Comet Hunter Jean-Dominique Maraldi in 1746, it languished in relative obscurity until Messier added it to his list in 1760, describing it as a “Nebula without stars”. Modern instruments show it as most definitely “with stars”, indeed there are several beautiful star chains visible through telescopes, as well as some deep, dark lanes and patches, adding to the “three-dimensionality” of the object, particularly in larger telescopes. There are quite a mix of older orange and newer blue stars within M2, making it a particularly pretty telescopic sight. 

Moving SW from M2, we arrive at three objects in quick succession: NGC 7009, The Saturn Nebula, the asterism M73 and another globular, M72. The Saturn Nebula is a fascination Planetary Nebula, well worth seeking out in any telescope, as it is reasonably bright, at +7.8 mag, yet compact at 0.5 arc minutes across. Telescopes of 6-8-inch aperture will be needed in order to see the two extended lobes that give the object its popular name. Lord Rosse, observing NGC 7009 in 1850, described two lobes or projections sitting either side of the nebula, making it appear very similar to Saturn, when its rings are edge on to us. Although the object has a distinctly un-Saturn-like green-blue hue, which is most easily seen in long duration photographs. The Saturn Nebula, in common with some other Planetaries - including the Blinking Planetary - can appear to blink on and o when looking at it for prolonged periods. This is of course a trick of the eye, caused by NGC 7009’s reasonably bright central star overwhelming a dark-adapted observers eye. When the observer averts their vision slightly, the Saturn Nebula returns to view. Although the Blinking Planetary is the most well-known object that exhibits this phenomenon, to the writer’s mind, the Saturn Nebula is actually the best example of a “Blinking” Planetary Nebula. As ever, aperture helps in resolving the finer details of NGC 7009 (especially the projections), but the Saturn Nebula should be sought out by all those with telescopes - it’s certainly bright enough to be seen in even the smallest scopes. 

The next object is an interesting one. When is a star cluster not a star cluster? Answer: when it’s an Asterism like M73. Lying less than 2 degrees SW of the Saturn Nebula, M73 has been the subject of some controversy over the years since its discovery. Charles Messier first noted it in 1780 as a “cluster of four stars with nebulosity”, although this nebulosity has never been picked up by any other observers. John Herschel, whilst including it in his General Catalogue, was suspicious of its definition as a true cluster. Debate raged on throughout the 20th century as to the true nature of the Y-shaped M73, with evidence of a relationship between the members of the group being published for and against. The matter was finally and conclusively put to bed in 2002, when spectral signatures of each of the constituent members, gathered in high resolution, concluded that they were all moving in different directions and the cluster was not, it fact, a cluster. M73 is not unique amongst the Messier list for controversial description, but remains interesting for the fact that it took so long to finally work out its true nature.

1.5 degrees to the west of M73 is the slightly less controversial Globular Cluster M72. At +9.27 mag, it is considerably fainter than M2, despite being not much smaller. M72 is considerably further away from us than M2 - it lies 55,000 light years distance from Earth. As it is fainter and further away, M72 requires a larger telescope to resolve individual stars. It is a pleasing sight in a 10-inch reflector and above, though William Herschel in his observing notes of 1783, noted that a power of 150x was needed to resolve the individual stars “fairly”.